1. Field of the Invention
The invention relates to a radio type selective calling receiver which accomplishes automatic gain control (AGC) in accordance with an electric field strength of received signals, and further to a method of receiving selective calling.
2. Description of the Related Art
A demand for pager system has recently remarkably increased due to development in mobile communication, and hence, there has been increased frequencies transmitted to radio type selective calling receivers from base stations. A radio type selective calling receiver includes an antenna having a wide band characteristic for receiving a number of frequencies, and picking up a desired signal and modulating it in sequential stages.
If a frequency is in connection with one wave, a receiver characteristic or IM characteristic is not harmfully influenced, even if a received signal had a great electric field strength. However, if a frequency is in connection with a plurality of waves, there would occur spurious due to saturation in a circuit, which would exert harmful influence on the receiver characteristic or IM characteristic.
In order to avoid such spurious, a radio type selective calling receiver is usually designed to accomplish automatic gain control (AGC) for detecting an electric field strength of a received signal, adjusting a gain of a front end thereof, and improving a receiver characteristic or IM characteristic.
A detector for detecting an electric field strength of a received signal detects amplitudes of intermediate frequency (IF) signals from respective stages of a multi-stage limiting amplifier which limits IF signals indispensable for FM signals, smoothes the thus detected amplitudes by means of a rectifier circuit, and converts into a dc voltage in accordance with a relation between an electric field strength and a dc voltage in a received signal. In AGC, the thus converted dc voltage is used for controlling a gain of a front end to thereby improve a receiver characteristic or IM characteristic.
However, a conventional radio type selective calling receiver is accompanied with a problem, as follows. Since the above-mentioned detector detects amplitudes of IF signals from respective stages of a multi-stage limiting amplifier, converts IF signals into a current by means of a rectifier circuit, totally adds the thus converted current, and obtains a voltage by passing the current through a resistor, the detector could have a quite small ability for providing a current. Hence, if the detector would provide a current to a circuit which accomplishes AGC, there is a fear that a voltage for operating the selective calling receiver might be dropped.
Japanese Unexamined Patent Publication No. 6-213676 having been published on Aug. 5, 1994, based on U.S. patent application Ser. No. 974631 having the filing date of Nov. 23, 1992, has suggested an indicator for indicating a strength of a received signal. The suggested indicator is comprised of a plurality of voltage amplifiers, a plurality of rectifiers, a plurality of voltage to current converters, and a current adding circuit. Each of the voltage amplifiers includes a pair of differential transistors driven by a first current which is in proportion to a temperature in order to keep a gain constant. Each of the voltage to current converters includes a pair of second differential transistors driven by a second current in proportion to a temperature, but in inverse proportion to a process parameter, and a third transistor driven by a third current in inverse proportion to a process parameter.
Japanese Unexamined Patent Publication No. 8-139632 having been published on May 31, 1996 has suggested a narrow band transceiver capable of operating AGC at a high speed without malfunction. The suggested transceiver is comprised of a receiver, a digital signal processor for demodulating an output signal transmitted from the receiver and transmitting a signal produced by modulating audio inputs, a transmitter, a detector for detecting an electric field strength of an output transmitted from the receiver, an indicator for indicating whether the transceiver is in receiving or transmitting mode, and a power controller for controlling power to be supplied to components of the transceiver in accordance with outputs transmitted from the detector and the indicator.
Japanese Unexamined Patent Publication No. 8-307172 having been published on Nov. 22, 1996 has suggested an amplifier having a variable gain. The suggested amplifier is comprised of a differential amplifying circuit having a gain variable in accordance with a bias voltage, transistors included in the differential amplifying circuit for amplification, and a circuit for compensating for temperature characteristic of gains of the transistors.
Japanese Unexamined Patent Publication No. 9-223950 having been published on Aug. 26, 1997 has suggested VCO circuit including a ring oscillator. A voltage to be applied to VCO circuit is inverted into a constant current signal by virtue of a saturation region of output characteristic of a field effect transistor. The thus produced constant current signal is supplied to the ring oscillator for driving.
In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide a radio type selective calling receiver capable of operating an AGC circuit without a drop in an operating voltage.
In one aspect, there is provided a radio type selective calling receiver including (a) a detector for detecting an electric field strength of a received radio signal, based on signals having an intermediate frequency, (b) a converter for converting a voltage associated with the thus detected electric field strength into a current, and (c) an automatic gain control circuit for receiving an output transmitted from the converter.
It is preferable that the selective calling receiver is of either super heterodyne type or direct conversion type for receiving a radio signal.
It is preferable that the radio type selective calling receiver further includes a power source which provides an intermittent voltage to the converter.
It is preferable that the radio type selective calling receiver further includes a dc voltage source, and a controller providing an intermittent voltage to the converter by applying intermittent control signals to a dc voltage supplied from the dc voltage source.
It is preferable that the radio type selective calling receiver further includes a dc voltage source, a controller, and a stabilizer, the controller providing an intermittent voltage to the converter by applying intermittent control signals to a dc voltage supplied from the dc voltage source, the stabilizer providing an intermitted voltage to elements other than the converter.
There is further provided a radio type selective calling receiver (a) a front end for converting a received signal into a first signal having an intermediate frequency, (b) a detector for detecting an electric field strength, based on the first signal, and demodulating the first signal, (c) a decoder for decoding the thus demodulated first signal, (d) a voltage to current converter for converting a voltage defined by the electric field strength into a current, and (e) an automatic gain controller for receiving the voltage from the voltage to current converter and controlling a gain of the front end.
In another aspect of the invention, there is provided a method of receiving selective calling, including the steps of (a) detecting an electric field strength of a received radio signal, (b) converting a voltage associated with the thus detected electric field strength into a current, and (c) transmitting the voltage to an automatic gain control circuit.
It is preferable that the voltage is converted into a current in accordance with an intermittent voltage.
In accordance with the present invention, it is possible to drive an automatic gain control (AGC) circuit without a drop in input signals.
In addition, the use of an intermittent voltage for driving the voltage to current converter makes it possible to use a single circuit battery as a power source.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.